A core in a nuclear reactor comprises a plurality of vertically arranged fuel assemblies. A fuel assembly comprises one or more fuel bundles. A fuel bundle comprises a plurality of vertical fuel rods arranged between a bottom tie plate and a top tie plate. The fuel rods contain a column of circularly cylindrical pellets of a nuclear fuel arranged in a cladding tube. At the bottom of the fuel rods, a bottom plug is arranged for insertion into holes provided therefor in the bottom tie plate, and at the top of the fuel rods, a top plug is arranged for insertion into holes provided therefor in the top tie plate. A small number of so-called supporting fuel rods retain the fuel bundle and are fixed to the bottom tie plate and the top tie plate. The fuel bundles or the fuel bundle are/is surrounded by a fuel channel which is normally designed with a square cross section.
During the burnup of the nuclear fuel, fission gases, which are contained within the fuel rod, are released. To prevent the pressure on the cladding from becoming too large, an expansion space is needed for the fission gases, a so-called fission gas plenum. The fission gas plenum should correspond to 5-10% of the volume of the fuel. For a fuel rod whose diameter is substantially constant and whose active length is 4 m, this means that the fission gas plenum should be 0.2-0.4 m. In full-length fuel rods, the fission gas plenum is arranged in the upper part thereof, above the reactor core, and hence exerts a minimum influence on the efficiency of the reactor. The active length of a fuel rod is the length of that part of the fuel rod which contains fuel pellets and does not comprise a fission gas plenum.
The core is immersed into water which serves both as coolant and as neutron moderator. During operation, part of the water changes into steam. At the lower part of the fuel assembly, there is only water, but the higher up in the fuel assembly, the higher the percentage of steam. When the percentage of steam rises, the neutron moderation deteriorates since steam is inferior to water as moderator. The reactivity of the reactor depends on the ratio between fuel and moderator. To improve the reactivity, the water/steam ratio in the upper part of the fuel assembly must be increased.
One way of obtaining a higher water/fuel ratio in the upper part of the fuel assembly is to replace the majority of the fuel rods with fuel rods which are tapering in an axial direction. The General Electric patent application EP-0 514 121 A1 discloses a fuel rod which comprises a lower column of fuel pellets with a relatively larger diameter, and an upper column of fuel pellets with a relatively smaller diameter and a cladding tube, which comprises a lower coarser part which surrounds the lower column of fuel pellets, an upper narrower part which surrounds the upper column of fuel pellets, and a transition portion between the lower and upper parts. The fuel rod described in the above EP publication has two fission gas plenums, one in the upper part of the fuel rod and one in the lower part of the fuel rod.
To obtain an optimum fuel-to-moderator ratio with tapering fuel rods, all or at least the majority of the fuel rods should consist of tapering fuel rods. The disadvantage of tapering fuel rods is that it is expensive to manufacture fuel pellets with two different diameters and tapering cladding tubes compared with ordinary fuel rods of uniform thickness where all the fuel pellets have the same diameter and the cladding tube is straight. Since the majority of the fuel rods consist of tapering fuel rods, the total cost of the fuel assembly is high.
Another method of reducing the fuel quantity in the upper part of a fuel assembly is to replace some of the fuel rods with part-length fuel rods. Part-length fuel rods have a shorter axial extent than traditional full-length fuel rods. Patent document EP-0 336 203 B1 discloses a fuel assembly in which the majority of the fuel rods are full-length fuel rods, that is, they extend from the bottom tie plate to the top tie plate, and a minority of the fuel rods are part-length rods, that is, they extend from the bottom tie plate towards the top tie plate but terminate somewhat below the top tie plate. In order to retain the part-length rods, they are fixed to the bottom tie plate. The fixing can be made by providing the bottom plug with threads and screwing it to the bottom tie plate.
Inspection and service of the fuel assemblies are performed at regular intervals, among other things to detect and correct fuel damage as early as possible to prevent extensive leakage of fuel and fission products. The inspection and the service take place in a special pool to which the fuel assemblies have been moved. The fuel assemblies are only accessible from above so the fuel rods have to be lifted from above. The non-supporting full-length fuel rods can be easily lifted up since they are not fixed to the bottom tie plate. The fuel rods which are fixed to the bottom tie plate must be loosened therefrom, which may entail complications, especially when the fuel assembly becomes older and the bond may start jamming. A further problem which arises when the part-length fuel rods are to be lifted up is that it is difficult to reach them and to engage from above. Therefore, specially constructed tools are required to lift out the part-length fuel rods.
The above-mentioned EP application (EP-0 336 203 B1) discloses a part-length fuel rod which has been provided with an extension in the form of a screwed vane (swirl vane) which throws water droplets to the sides so that they hit adjacent fuel rods. In this way, an improved dryout margin is obtained. To attain an increase of the dryout margin, it is sufficient for the swirl vane to extend along the active length of the other fuel rods.
To obtain an optimum water/fuel ratio in the fuel assembly, the fuel column in a part-length fuel rod should have a certain optimum height. The optimum height of the fuel column may be obtained by calculations. A problem with part-length fuel rods is where to place fission gas plenums to give a minimum influence on the efficiency of the reactor. The above-mentioned EP application (EP-0 336 203 B1) discusses this problem in column 18, lines 52-58, and column 19, lines 5-22. One way is to arrange fission gas plenums in the upper part of the part-length fuel rods, that is, above the optimum height of the fuel column. However, placing fission gas plenums in the upper part of the part-length fuel rod entails several considerable drawbacks. One drawback is that part of the volume which could have been filled with a moderator cannot be utilized, and this in a region where moderators are already an in short supply. This solution also provides a number of relatively large fuel- and moderator-free volumes centrally in the core, which is not good, either from the neutron point of view, or from the efficiency point of view. As a better solution to the problem it is proposed that the whole, or at least a part of, the fission gas plenum should be placed at the bottom of the fuel rod. A disadvantage with this solution is that the total fuel quantity in the fuel rod decreases by 5-10%, which leads to an undesired reduction in efficiency.